There is increasing interest in using high strength titanium alloys as cast components in order to reduce costs compared to components machined from wrought forms. However, the large grain size frequently associated with as-cast titanium is detrimental to ductility, toughness and fatigue resistance compared with wrought products. Furthermore, grain size increases with increasing weight and section thickness of the cast component, resulting in increasing detriment to mechanical properties.
The problem of large grain size imposes severe restrictions on the potential of titanium alloy castings to compete structurally with components machined from wrought forms.
Grain refinement of aluminum castings by the addition of an inoculant has long been practiced by aluminum foundries. The most commonly used inoculant for aluminum-base alloys is Al.sub.3 Ti. Grain refinement of aluminum by titanium is due to the occurrence of a peritectic reaction at the aluminum-rich end of the aluminum-titanium phase diagram; see "Mechanism of Grain Refinement of Aluminum Alloys", Crossley and Mondolfo, Transactions AIME, Vol. 191, pp. 1143-1148 (1951). Briefly, the peritectic principle of grain refinement states that during cooling of the melt crystals of the primary phase form, which react peritectically with the liquid upon further cooling, the peritectic reaction transforms at least partially the primary crystals into crystals of the secondary phase, which then act as nuclei for solidification of the remaining melt. The least amount of titanium necessary for occurrence of the peritectic reaction in binary combination with aluminum under equilibrium conditions is 0.15 weight percent; see Hansen, "Constitution of Binary Alloys", 2d Ed., McGraw-Hill Book Co., p. 146 (1958). The principle of peritectic grain refinement has also been successfully applied to copper alloys; see, "Grain Refinement of Copper", Gould, Form and Wallace, Modern Castings, May 1980 and Transactions American Foundrymen's Society, Vol. 68, 1960. An alloying addition of iron to copper in sufficient amount to produce the peritectic reaction; i.e., 2.8 weight percent or more, produces grain refinement; see, Hansen, supra, p. 581.
Smaller amounts of iron added in powder form just before casting of the copper also produces grain refinement. In this case, the local concentrations exceed the requirement for the peritectic reaction, although the overall composition under homogeneous conditions does not. Therefore, localized nucleation occurs producing grain refinement.
A difficulty in applying the peretectic principle for grain refinement more generally is the rarity of alloying additions which form a peritectic reaction with the base metal at a sufficiently low solute concentration. At the present state of the art, no inoculants for the grain refinement of as-cast titanium alloys exist.
Carbon, nitrogen and oxygen are known to cause peritectic reactions with titanium. However, the minimum quantities of these additions needed to produce the peritectic reaction are associated with unacceptable embrittlement of titanium alloys. The amounts in weight percent are: 0.25 percent carbon, about 1.3 percent nitrogen and about 1.2 percent oxygen; see Hansen, supra, pp. 384, 990 and 1069. These amounts may be compared with the maximum amounts in weight percent found in commercial titanium alloys of 0.1 percent carbon, 0.07 percent nitrogen and 0.25 percent oxygen; see Metals Handbook, Vol. 3, 9th Ed., American Society for Metals, Metals Park, Ohio, p. 357 (1980). These limits, therefore, prohibit a direct application of the peritectic principal of grain refinement to titanium alloys.
Furthermore, several patents which are directed to as-cast titanium alloys containing up to 2 weight percent carbon do not mention or indicate any grain refining effect associated with carbon; see U.S. Pat. Nos. 2,818,338; 2,818,337; 2,818,336; 2,818,335; 2,818,334; 2,818,333 and 2,786,756.
Several patents do comment on the fact that carbon additions up to 0.25 weight percent and up to 0.3 weight percent contribute to fine grain size in wrought and recrystallinized titanium; see, U.S. Pat. Nos. 2,669,513 and 2,596,486. Consistent with the prior art, applicant has verified that 0.3 weight percent carbon additions to the titanium alloys Ti-6Al-4V (the most commonly utilized alloy) and Ti-2.7Al-13V-7Sn-2Zr (U.S. Pat. No. 3,986,868) had no beneficial effects on the as-cast grain size.